Embodiments of the inventive concepts relate to a method of fabricating a solar cell, more particularly, to methods of manufacturing a solar cell, in which a buffer layer is formed by vacuum evaporating technique.
Copper indium gallium selenide (CIGS) solar cells recently attracting a lot of interest exhibit good properties in terms of efficiency and stability (e.g., without no initial deterioration), compared with amorphous silicon solar cells, and thus, further research is being carried out to commercialize the CIGS solar cells. In addition, the CIGS solar cells achieve a specific power of about 100 W/kg that is definitely superior to the conventional silicon or GaAs solar cells of 20-40 W/kg. Furthermore, the CIGS solar cell of a single junction structure exhibits efficiency of about 20.3% that is substantially equivalent to the maximum efficiency, i.e., 20%, of the conventional silicon solar cell.
With all its advantages, there is a difficulty in fabricating the CIGS solar cells with high productivity. This is because there is no technology capable of continuously producing the CIGS solar cells. For example, the CIGS solar cells have been fabricated using a batch process requiring many manual tasks. A step of forming a buffer layer in a wet manner is a main reason for this difficulty. Despite its simplicity, the wet step cannot be continuously performed after or before a vacuum-required step, and this makes the fabricating process complex and time consuming Moreover, the wet step suffers from high cost for treating of waste matters. In this sense, a new method for forming the buffer layer is required to commercialize the CIGS solar cell.
In a conventional art, the buffer layer may be formed using a sputtering or co-deposition method. However, for the sputtering method, it is hard to form the buffer layer having good uniformity, because a sputtering target such as cadmium sulfide CdS, zinc sulfide ZnS or indium sulfide InS cannot be sputtered in chemical composition. The sputtering target is heated by heat generated from sputtering plasma. Sulfur (S) in the sputtering target has thermal conductivity different from other metallic elements (e.g., Cd, Zn, or In) of the sputtering target. This results in a serious reduction in reproducibility. For the co-deposition method, the formation of the buffer layer requires a step of evaporating a metal material at a temperature of 1200° C. or more, and thus, it is hard to supply a large amount of source material. As a result, it is hard to realize a solar cell having large area and high productivity.